Short length of pipe works better, much easier to aim................................................at least that's what I heard.
If a chopper was flying so low that it was "endangered" by a bottle rocket, it was too damned low to start with.
Shoot, now I've a diagram of an aimable bank of pipes dancing around in my head. Not for the assinigned purpose of shooting at helicopters of course, but it would be neat for the fourth of july, shooting up "strings" of bottle rockets.
A piece of pipe works a lot better, you get better aim and don't get burned.